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Porphyry and Other Molybdenum Deposits of Idaho and Montana

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Porphyry and Other Molybdenum Deposits of Idaho and Montana Porphyry and Other Molybdenum Deposits of Idaho and Montana Joseph E. Worthington Idaho Geological Survey University of Idaho Technical Report 07-3 Moscow, Idaho ISBN 1-55765-515-4 CONTENTS Introduction ................................................................................................ 1 Molybdenum Vein Deposits ...................................................................... 2 Tertiary Molybdenum Deposits ................................................................. 2 Little Falls—1 ............................................................................. 3 CUMO—2 .................................................................................. 3 Red Mountain Prospect—45 ...................................................... 3 Rocky Bar District—43 .............................................................. 3 West Eight Mile—37 .................................................................. 3 Devil’s Creek Prospect—46 ....................................................... 3 Walton—8 .................................................................................. 4 Ima—3 ........................................................................................ 4 Liver Peak (a.k.a. Goat Creek)—4 ............................................. 4 Bald Butte—5 ............................................................................. 5 Big Ben—6 ................................................................................. 6 Emigrant Gulch—7 .................................................................... 6 Sawtooth Batholith—40 ............................................................. 6 Laramide Molybdenum Deposits ............................................................... 7 Intrusion-Hosted Deposits .................................................................. 7 Kaniksu Batholith Cluster—42 .................................................. 7 International Molybdenum Company—42a ....................... 7 American Girl—42b ........................................................... 7 Hotchkiss (Hines)—42c ..................................................... 7 Spring Creek—36 ....................................................................... 7 Bentz—9 ..................................................................................... 8 Miners Gulch—10 ...................................................................... 8 Henderson Gulch—11 ................................................................ 8 Black Pine Mine—12 ................................................................. 8 North Flint Creek Molybdenum Cluster—13, 14, and 15 .......... 8 Tunstill—13 ........................................................................ 8 Tolean Lake—14 ................................................................ 8 East Goat Mountain—15 .................................................... 8 Black Lion Mountain—16 .......................................................... 8 Hecla District—41 ...................................................................... 9 Elkhorn Mine—17 ...................................................................... 9 Monaghan—39 ........................................................................... 9 Thompson Park—18 ................................................................... 9 Golden Sunlight—19 .................................................................. 9 Turnley Ridge—20 ..................................................................... 9 Granite Peak—21 ..................................................................... 10 iii Gold Bug-Hawkeye—22 .......................................................... 10 Judith Peak-Red Mountain—23 ............................................... 10 Sediment-Hosted Deposits ............................................................... 10 Cannivan Gulch—26 ................................................................ 10 Stewart Gulch—27 ................................................................... 10 Belt Terrane Molybdenum Deposits ................................................ 11 Stonehorse—28 ........................................................................ 11 Armor Creek (a.k.a. Odell Mountain)—29 .............................. 11 Tim Creek—30 ......................................................................... 11 Black Pine Prospect—31 .......................................................... 11 Ramona Creek—32 .................................................................. 11 Eight Mile Creek—33 .............................................................. 11 Chilco Mountain (a.k.a. Pyrite Creek)—34 ............................. 11 Pre-Laramide Molybdenum Deposits ...................................................... 12 Thompson Creek—35 .............................................................. 12 Little Boulder Creek (a.k.a. White Cloud Peak)—24 .............. 14 Ramey Creek—25 .................................................................... 14 Cabin Creek Prospect—44 ....................................................... 14 Virginia-Beth—38 .................................................................... 14 General Discussion .................................................................................. 15 Evaluation of Outcrops .................................................................... 15 Classification of Molybdenum Deposits .......................................... 15 Ages of Molybdenum Deposits ........................................................ 15 Tertiary ..................................................................................... 15 Laramide ................................................................................... 16 Pre-Laramide ............................................................................ 17 Conclusions and Exploration Priorities ................................................... 17 Acknowledgments .................................................................................... 18 References ................................................................................................ 18 Table 1. Molybdenum Deposits of Idaho and Montana ............................ 2 Figure 1. Porphyry molybdenum deposits of Idaho and Montana ............. 4 Figure 2. Porphyry molybdenum deposits of Idaho and Montana, showing deposit types ........................................................................ 5 iv Porphyry and Other Molybdenum Deposits of Idaho and Montana Joseph E. Worthington1 INTRODUCTION rences, such as the older (Jurassic) occurrences of Washington and Adams counties of Idaho, are excluded, even though they may contain molybdenum. The term The present review is the result of about forty years of porphyry deposit in this discussion refers to a general type intermittent association with, study of, and exploration for, of large, bulk minable deposit, following usage in the porphyry molybdenum deposits. Although the subject has A.I.M.E. Porphyry Copper Volume (Parsons, 1933, p. been of little interest for the past decade or so, price 338-340). Parsons explains that the word “porphyry” as a increases since mid-2004 have greatly renewed interest in deposit name came from the first four deposits that were molybdenum. No attempt has been made at an exhaustive developed, which all contained prominent porphyritic library search, nor has there been any comprehensive rocks. Several that were developed later contained the attempt to collect data on all current or recent programs. bulk of their mineralization in wall rock, but the term “porphyry” was retained. It is well recognized that all so- The area under review includes the batholithic terrane called porphyry deposits do not contain porphyritic rocks, of central Idaho and Montana, and comprises, perhaps, and many in this discussion certainly do not. Many, in 100,000 square miles. It would be delimited by the inter- fact, are listed merely as occurrences that could be national boundary to the north, the Cenozoic volcanics of examined further. Discoveries have been made in this the Columbia River, the Snake River, and the Yellowstone manner, as will be discussed subsequently. Plateau on the west and south, and the Rocky Mountain thrust front and nearby plutons to the east. The area is Nearly fifty molybdenum deposits are described with made up of the Idaho batholith and younger plutons in locations, ages (if available), brief details on rock Idaho and the Boulder, Pioneer, and other large plutons composition, and literature references. Deposits are listed plus smaller and younger plutons in Montana. The hosts to in Table 1, each with a location number that, in turn, is these plutons are folded and thrust faulted late- repeated in its heading in the text and corresponds to the Precambrian and Phanerozoic sediments with lesser location number on Figures 1 and 2. The ages are taken exposures of older Precambrian basement. There are also from research sources, and most are K/Ar dates. Where no areas of Tertiary and recent cover, including volcanics and dates are available, a stratigraphic age may be assigned, if sediments, in both states. Much of the region is underlain it is obvious, or an analogy to a nearby date may be cited. by the late Precambrian Belt sedimentary basin. The Many of the deposits considered are of porphyry molybdenum deposits appear to be restricted in age from affiliation, using criteria cited above, and others are only Late Cretaceous to Eocene (98 m.y. to 36 m.y. using
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